The site of integration of the polyoma genome in transformed Syrian hamster cells will be characterized. Cell DNA from independently transformed hamster cells will be digested with site-specific restriction endonucleases, and the topology of the polyoma genome relative to the adjoining cell sequences will be examined. DNAs containing viral and cellular sequences will be joined with bacterial plasmids, cloned in bacteria and amplified in sufficient quantities to permit chemical and biological characterization of the viral genome associated with the cell sequences. Excision of the viral genome in inducible cell lines transformed by the ts-A mutant will be further characterized.